Create a 2:1 mux that behaves normally when sel is 0 or 1, but if sel is x/z in simulation, assert sel_unknown=1 and clamp y to 8'h00.
sel
0
1
sel_unknown=1
y
8'h00
Requirements:
safe_mux2
a[7:0]
b[7:0]
y[7:0]
sel_unknown
